1. Field of the Invention
This invention relates to a pneumatic radial tire requiring high running performance or simultaneously satisfying dry and wet running performance on a circuit course or the like.
2. Related Art Statement
As shown in FIG. 4, the conventional tire having high running performance has a so-called unidirectional tread pattern comprising a combination of plural circumferential straight grooves 31 (four grooves in the illustrated embodiment) and slant grooves 32 slantly extending across the circumferential straight groove 31 located in the side end portion of the tread and approaching a lower end viewed at a posture mounted onto a vehicle to the central portion of the tread toward the rotational direction of the tire. An inclination angle .theta. of each of the slant grooves 32 with respect to a meridional line Y--Y of the tire becomes larger at the side of the central portion of the tread. Such a tire can develop excellent wet running performance based on the four circumferential straight grooves 31 and slant grooves 32.
When this tire is used on a circuit course or the like for sport running, a tread side region of the tire located outward the vehicle at the mounted state, for example, a right half region of the tire shown in FIG. 4 is particularly subjected to a large reaction force from road surface in the turning of the vehicle.
As a result, each of island units 33a defined by the circumferential straight groove 31 and two slant grooves 32 and constituting a rib 33 located from the circumferential straight groove 31 toward the central portion of the tread largely wears at a side toward the tread side portion as shown by a shadowed region in FIG. 5a. In this case, the worn region and its worn depth become large at the stepping-in side of the island unit 33a, which tends to become vigorous as the angle .theta. of the slant groove 32 with respect to the meridional line Y--Y of the tire becomes large for the purpose of improving the wet performance of the tire. According to such a conventional tire, therefore, the effective ground contacting area of the rib 33 is lowered to bring about the poor ground contacting and the like, and hence the steering stability is undesirably and considerably lowered.
In order to reduce the worn amount of the island unit 33a at the step-in side, there is proposed a technique that a sawtooth zigzag groove 34 is arranged at the side portion of the tread instead of the circumferential straight groove 31 while maintaining the angle of the slant groove 32 with respect to the meridional line of the tire at a required value to make a difference between angles of corners at stepping-in side A and kicking-out side B of the island unit 33a small as shown in FIG. 5b. In this case, the difference in the wearing degree between the stepping-in side and the kicking-out side of the island unit 33a is advantageously reduced as shown by shadowed region in FIG. 5b.
According to this technique, however, the drainage performance through the sawtoothed zigzag groove 34 becomes smaller than that of the conventional circumferential straight groove 31, and consequently the wet running performance of the tire are undesirably lowered to cause a fear of generating hydroplaining phenomenon.